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sample 0 ms submission
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {

    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1==null){
            return t2;
        }
        if(t2==null){
            return t1;
        }
        t1.val=t1.val+t2.val;
        t1.left=mergeTrees(t1.left,t2.left);
        t1.right=mergeTrees(t1.right,t2.right);
        return t1;
    }
}
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sample 40460 kb submission
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if(t1 == null) {return t2;}
        if(t2 == null) {return t1;}
        t1.val += t2.val;
        t1.left = mergeTrees(t1.left , t2.left);
        t1.right = mergeTrees(t1.right , t2.right);
        return t1;
        
    }
}
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